Format mouse, to sum selective cells

Short background: I am 83 year old Excel user. I have taught some basic workshops several years ago. I used it in my career as a proposal coordinator, aerospace proposals, aircraft engines and road and bridges.

However, today I decided to analyze my monthly expenses. I am using Windows 7 and Excel 2003.

I have a column with about 50 cells of various dollar amounts.
I use to point my mouse to the lower right corner of the worksheet just about time in the system tray.
Then I would right key the mouse, options would display such as count, average, sum and I would select sum.
I select sum.
Then I touch control and select the first cell. Nothing displayed in the sum area
I continue to hold down control and select second cell. Immediately, the sum for the 2 cells are in the sum area.

This is my problem. When I right key, nothing happens.

Any suggestions? Thank you